The correct answer is (C) To investigate. To search, as if burrowing; to perform thorough research (of documents, for data or information, etc.). Also, to dig, as with a spade; to excavate; to obtain by digging.

Delve comes from Middle English delven, Old English delfan (to dig).

Examples of use:

• She immediately delved into the problem.
• He intended to delve into all possible sources during his search.